const {createElement, useLayoutEffect, useState} = React;

const {createRoot} = ReactDOM;

function App() {

  const [isMounted, setIsMounted] = useState(false);

  useLayoutEffect(() => {

    setIsMounted(true);

  }, []);

  return createElement('div', null, `isMounted? ${isMounted}`);

}

const container = document.getElementById('container');

const root = createRoot(container);

root.render(createElement(App));